Firstly, the configuration of a device including an interface which is to be connected over a serial bus of IEEE 1394 (IEEE std. 1394–1995 IEEE standard for a High Performance Serial Bus) will be briefly described. FIG. 10 is a schematic view showing the configuration of an electronic device (termed as IEEE 1394 device) which is connected via a serial bus of IEEE 1394 standard (complying with P1394a). Devices which can be connected via the IEEE 1394 serial bus, such as computers installed in a consumer's electric appliance, an automobile, a measuring instrument, an electronic music instrument, a personal computer and etc., may constitute the IEEE 1394 device.
Referring now to FIG. 10, the IEEE 1394 device 1 comprises, as an interface device (IEEE 1394 controller) for performing communication with other nodes over an IEEE 1394 serial bus 11, a physical layer 10 referred to as “PHY”), a link layer 21 and a higher level CPU 22, etc. The physical layer (PHY) 10 usually comprises an LSI (large scale integrated circuit) and has various functions such as bus initialization, bus arbitration, data re-synchronization, coding/decoding of transmit and receive data, connection state management and port control. The CPU 22 is adapted to perform control of the application layers. The IEEE 1394 serial bus 11 is connected electrically by a point to point basis and a connection topology of a plurality of the IEEE 1394 serial buses is based on star type topology. The IEEE 1394 serial bus 11 comprises a 6-core cables including two-pairs of shielded twisted pair cables and one pair of power supply cable (two pairs of twisted cables comprising 4 cores may be used) as a standard bus cable. One of the two pairs of twisted pair cable is used for data transmission, while the other pair is used for transmission of strobe signal.
As for a publication with regards to the reduction in power consumption of an electronic device which is connected via IEEE 1394 serial bus, there exists, for example, JP-A No. 10-70561, which proposes a method of eliminating a problem that when a power supply is turned on at each node, an electric power is supplied to all blocks such as a physical layer, link layer and higher level CPU, so that the electric current is consumed wastefully in these blocks even if the node is not connected to other nodes. This method comprises the steps of causing the link controller not to operate when the node is not connected to other node via IEEE 1394 serial bus and causing the controller to operate when the node is connected thereto so that the power consumption in a communication interface is reduced.
Referring to FIG. 10, as a low power consumption feature, a circuit including a Suspended/Resume function for the physical layer 10, link layer 21 and the higher level CPU is partially standardized as a specification (P1394a. Draft 3.0, which are an additional standard of IEEE Std. 1394 to 1995). In this additional standard, states such as “Disabled”, “Disconnected” and “Suspended” are defined in order to reduce the power consumption of the physical layer.
FIG. 11 is a diagram showing the configuration of a high speed serial bus and an IEEE 1394 long-distance device for performing long distance transmission (also referred to as “Giga bits 1394” which is prescribed in the additional standard P1394b). Referring now to FIG. 11, the physical layer (PHY) 10 of one node outputs a signal to an optical fiber via an optical transceiver 30 which is connected to an optical port and transmits a signal to an optical transceiver (not shown) of the physical layer of the other node and receives via the optical transceiver 30 a signal which is output to the optical fiber 12 from the optical transceiver (not shown) of the physical layer of the other node. Some of IEEE 1394 long distance devices for performing long distance transmission use unshielded twisted pair, the specification of which is specified by the IEEE 1394 standard P1394b (category 5 UTP physical medium dependent layer; CAT-5 UTP: category 5 Unshielded Twisted Pair).
FIG. 12 is a diagram showing the configuration of the physical layer 10 of the IEEE 1394 long distance device shown in FIG. 11. Referring to FIG. 12, the physical layer 10 of the IEEE 1394 long distance device comprises a main logic 100, a plurality of transmission/reception ports 110 (N transmission/reception ports in FIG. 12) and a link interface 120. The main logic 100 comprises a tone generating unit 10 for generating a tone signal (TONE), a connection management state machine 102 for managing the connection state for each of the transmission/reception ports, a transmission/reception circuit 103 for encoding and decoding of transmission/reception data and an arbitration state machine 104 for managing the arbitration of buses. The transmission/reception ports 110-1 of the plurality of transmission/reception port 110-1 to 110-8 is connected to the optical transceiver 30.
The optical transceiver 30 performs conversion from an electrical signal to an optical signal of the transmission signal, conversion of a received optical signal to an electrical signal and reporting of optical detection, etc. As schematically shown in FIG. 13, the optical transceiver 30 comprises a transmit circuit 31 which receives data from the transmission port, an E/O (electrical-to-optical) conversion circuit 36 comprising a semiconductor laser for converting electrical signal into optical signal, an O/E (optical-to-electrical) conversion circuit 37 comprising a photodiode for converting an optical signal which is received via the optical fiber (12 in FIG. 11) to electric signal and a reception signal which receives an electric signal from the O/E conversion circuit 37 to output to the transmission/reception port. The receiver circuit 32 comprises a current-voltage conversion circuit which receives a detection current of the O/E conversion circuit 37 and converts it into a voltage and a buffer circuit which receives an output from the current-voltage conversion circuit and outputs a voltage. The receiver circuit 32 comprises a signal detecting circuit (not shown) which outputs a signal detection signal (SD) (Signal Detection) to the physical layer 10 on detection of the signal. If the data is transmitted on parallel-bits basis by using the reception port and the optical transceiver 30, the optical transceiver 30 includes a parallel to serial conversion circuit and a serial to parallel conversion circuit.
FIG. 14 is diagram for explaining the state transition in the physical layer 10 of the IEEE 1394 long distance device. A data signal line, which transmits and receives data, is provided between the transmission/reception ports of the physical layer 10 (corresponding to 110 in FIG. 12). Further a signal detection signal (SD) line for notifying the physical layer 10 when the signal is detected by the signal detecting circuit of the optical transceiver 30 and a plug signal (PLUG) line which notifies the connection state between the connector (not shown) of the optical transceiver 30 and the cable connector are connected therebetween. In other words, when the plug (or receptacle) of the connector (not shown) of the optical transceiver 30 is connected to the cable connector, the plug signal (PLUG) is rendered active (“1”).
When the physical layer is in the disconnected state and a cable is connected to the connector of the optical transceiver 30 , the plug signal (PLUG) is rendered active “1” and the physical layer 10 performs an initialization operation (speed arbitration operation) by tone signal transmission. If there is no destination node, the plug signal remains “disconnected” and a connection monitoring tone signal (TONE) is transmitted (refer to FIG. 6). If there is a destination node to be connected, the state transition to a Resuming state is executed for the physical layer to perform operations such as transmission of a continuous signal. If a handshake with a destination node is normally conducted, the state transition to the active state is executed.
When the physical layer is in an active state and receives an instruction of suspending the transmission/reception port from a higher level device which controls the physical layer (Suspended Initiator), or when the destination note is brought into a suspended state (Suspended target), the transmission/reception port in interest is brought into a “Suspended” state.
When the physical layer 10 is in a suspended state and receives a resuming instruction from the higher level device or the optical transceiver of its own node receives the signal detection from another node (at this time, the signal detection signal (SD) of the optical transceiver is rendered active), the transmission/reception port of the physical layer in interest is returned to a Resuming process. When the suspend instruction issued from the higher level device is received by the physical layer 10 in the resuming process, the physical layer 10 is brought into a suspended state.